Generally, the lifespan of engine oil depends on a running distance of a vehicle or operating time of the engine. As time passes, engine oil is oxidized or nitrified, and deteriorates by being mixed with carbon soots, metallic particles, fuel oil or coolant. Various apparatus for detecting engine oil deterioration are required, which are usually costly for both manufacturers and consumers. There is a need for a primary apparatus for detecting oil deterioration in an effective manner. When detecting diesel engine oil deterioration, measurement of carbon soot content in oil is most preferable.
Diesel fuel is mostly burned in a combustion chamber. A portion of the fuel is unburned and subject to thermal decomposition under high temperature. Unburned fuel components generate carbon soot particles. A portion of the carbon particles is discharged to the atmosphere by exhaust fumes, and the portion remaining in the combustion chamber is mixed with engine oil. Carbon particles in the engine oil are transformed into soot lumps by reacting with engine additives such as dispersing agent or detergent. The soot lumps reduce flow rate of the engine oil by increasing its viscosity, and cause excessive wear on the surfaces of engine components like cylinders, bearings, and the like, thereby deteriorating engine performance considerably. Accordingly, the oil change maintenance period depends largely on the carbon soot content.
Allowable carbon soot content in diesel engine oil is in the range of about 0.5% to 4%. Carbon soot generated in a diesel engine is a very minute spherical particle ranging from several tens of nanometers to dozens of micrometers, and is impossible to filter with conventional oil filters.
Several apparatus for measuring quantitative carbon soot content in oil are known, and a thermal gravimetric analyzer is most widely used. As a thermal gravimetric analyzer is used to test small amounts of sample oil extracted from the engine, it is incapable of measuring oil contamination in real time.
The state of the art apparatus for measuring carbon soot content in real time measures variation of dielectric constant of the oil.
An example of an apparatus for measuring soot content using variation of dielectric constant of oil is the fluid condition monitor disclosed in U.S. Pat. No. 6,278,281 issued to Bauer et al. The method of monitoring fluid condition comprises the steps of immersing a pair of electrodes in the fluid; applying an oscillating, substantially constant peak supply voltage sequentially at a first frequency of at least one hertz and at a second frequency of less than one hertz across the electrodes; measuring the current at the first frequency representative of the bulk impedance of the fluid and the second frequency representative of the surface impedance of the electrodes, and calculating the difference of the measured currents; and comparing the difference in measured currents with a reference level for a predetermined acceptable fluid condition.
The apparatus of U.S. Pat. No. 6,278,281 which uses variation of dielectric constant in oil, reacts very sensitively to oil oxidization, water, metallic wear particles mixed with the oil as well as the concentration of carbon particles. This apparatus, therefore, cannot accurately measure the carbon soot content in oil. Additionally, the apparatus cannot monitor oil condition accurately since it is sensitive to electrical disturbances and dependent on temperature, which are common problems with most apparatus depending on electric characteristics of the oil.
Examples of apparatus for measuring oil contamination with emitted light in oil are described in U.S. Pat. No. 4,699,509 issued to Kamiya et al., and U.S. Pat. No. 5,548,393 issued to Nozawa et al.
U.S. Pat. No. 4,699,509 discloses a device for measuring contamination of lubricant, in which an optical path gap is provided between a light source window at a light source side and a light receiving window at a light receiving element side. A lubricant having an amount of contaminant to be measured in accordance with light transmittance is present in the optical path gap, and the length of the optical path gap is dozens of micrometers.
It is difficult for lubricant to flow smoothly through such a short optical path gap because of flow resistance due to viscosity of the lubricant, and the length of the optical path gap must be adjusted accurately and maintained constant.
The oil deterioration detecting apparatus of U.S. Pat. No. 5,548,393 applies the “Goos-Hänchen shift” phenomenon. When light advances from a first medium with a first refractive index to a second medium with a second refractive index at an angle larger than a total reflection critical angle, the light will not enter the second medium with the second refractive index, and will be totally reflected. The reflection light goes out from a point apart from an incidence point by a certain distance, and the distance is called “Goos-Hänchen shift.”
The apparatus of U.S. Pat. No. 5,548,393 comprises a prism having a boundary surface in contact with the liquid to be examined; a light-emitting portion for emitting light toward the boundary surface; and a photosensor for receiving the light reflected by the boundary surface with “Goos-Hänchen shift” and for outputting a signal corresponding to the amount of the received light. The light power changes in accordance with the concentration of carbon particles in the oil.
In the apparatus of U.S. Pat. No. 5,548,393, the boundary surface in contact with the liquid to be examined is limited to one or three, and precise measurement can not be achieved for low level of contamination. Also, this prior art apparatus ignores the possibility that refractive index of the liquid may increase as the soot content in the liquid increases. In other words, when the soot content in the liquid increases, the critical angle at which the light is totally reflected by the boundary surface, also increases more than when the liquid is clean. Therefore, the incidence angle of light becomes smaller than the critical angle, thereby violating the total reflection condition. Accordingly, the power of light reflected by the boundary surface decreases considerably, and the oil deterioration is exaggerated and erroneously detected as if the soot content in the oil were very high.
Furthermore, the above prior art apparatus further comprises additional complicated components such as ball-shaped cleaning members disposed in the liquid which are moved throughout the liquid and contact the boundary surface, and flow-out prevention members adapted to prevent the cleaning members from flowing away from a location near the boundary surface. Because the contaminant adheres to the surfaces of the cleaning members as well as the boundary surface in the diesel engine oil having a high soot content, the cleaning effect by the members is very small. And, further problems are introduced when the contaminated cleaning members have to be replaced with fresh members. It is difficult to determine the appropriate exchange times for such cleaning members.